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Efficiency breakthrough in solar thermal cells

01/01/2013

A conventional PV/T cogen installation in the UK Credit: Alfagy

Solar power researchers in the US and Canada claim a paradigm-shifting breakthrough will improve both the electrical and thermal efficiencies of thin-film hybrid photovoltaic-thermal (PV/T) devices, reports Robert Stokes.

Although thin-film photovoltaic (PV) cells offer the most usable solar energy per square metre of roof space, using them for photovoltaic-thermal (PV/T) cogeneration has faced major obstacles.

These issues now appear to have been solved, opening up welcome diversification strategies for PV/T companies themselves and for hard-pressed makers of thin-film PV arrays that trap light and turn it into electricity. Not only that, by publishing practical solutions without patenting key techniques, the academic in charge of the research programme has put them into the public domain, making them free for anyone to use as they see fit.

Hybrid PV/T systems generate electricity, heat and, sometimes, cooling, a state of affairs that should have any disciple of cogeneration and trigeneration thrilling to the ecstasy of a two- or even three-fold 'higher efficiency'.

So why are they not ubiquitous on the roofs of homes, business and municipal buildings in the world's sunniest locations, and other places where they could provide both energy and cost savings? One reason is that the electrical efficiency of conventional PV cells formed from crystalline silicon (c-Si) – the material of choice for most existing PV/T systems today – degrades by around 0.04% per degree Celsius. As they get hotter they get worse at converting light into power.

Standard PV/T systems counter this by cooling their PV cells, but this naturally makes for terrible thermal efficiency if the systems are also used for heating. So PV/T systems built around c-Si PV tend generally to be valued for conversion of light into electricity.

'It was never a good marriage between c-Si and solar thermal,' says Dr. Joshua Pearce, professor at the Michigan Technological University, Houghton, MI, US, in which role he is cross-appointed in both the Department of Materials Science and Engineering and the Department of Electrical and Computer Engineering.

Higher efficiencies

Earlier this year, Dr. Pearce and collaborators published the results of their groundbreaking research that holds out the promise of hybrid PV/T systems with significantly enhanced electrical efficiencies.

Since the late 1990s, it has been known that substituting amorphous silicon (a-Si) for c-Si might get around the degradation constraint as a-Si electrical efficiency reduces by only 0.01% per degree Celsius. This, it was widely held, could be the basis of PV/T systems running at higher temperatures than those built around c-Si. But the catch was that until Pearce and his fellows took another look, a-Si seemed bound to run up against a physical barrier, the exotically named Staebler–Wronski effect.

With the data freely available, people can now make informed decisions when considering a PV/T system for their home Credit: Alfagy

Staebler–Wronski describes a trade-off in which exposing a-Si PV to light produces progressive degradation of the cells' electrical efficiency until this reaches a more or less steady base level known as the 'degraded steady state' (DSS). By this time it may have lost 10% of its overall efficiency.

Pearce and others showed over the past decade that this happened after about 100 continuous hours of sunlight in the case of the highest quality a-Si PV, not a long time in some sunny places.

It had become clear, though, that Staebler–Wronski need not be an insurmountable barrier to more efficient hybrid PV/T.Research had demonstrated that a degraded a-Si PV cell could get its mojo back, so to speak. It could be restored to its initial efficiency by being heated to 150°C for four hours, during which a process called annealing – affecting the physical and electrical properties of the silicon – takes place spontaneously.

Armed with this knowledge, PV researchers suggested various ways to exploit it. One was to remove an array of a-Si PV cells once they had degraded in light, then heat them up to a moderately high temperature, say 80°C, for a long time in an air oven.

As Pearce points out, though, this would be somewhat sub-optimal in practice. Better to do the baking in situ, and what better way to do this than to combine solar cells into a PV/T system, where heat generation was anyhow part of the big picture?

One critical observation in pursuit of this ideal was that a-Si PV cells are more efficient at high temperatures such as the 100°C that could be reached in a solar thermal flat-plate collector. This suggested an interesting possibility. Why not deposit a-Si right there on the solar thermal absorber plate in a PV/T system and let the high temperatures reached facilitate annealing in deliberately pulsed cycles (spike annealing) that would supercharge the efficiency with which cells turned light into electricity.

This is what Pearce and colleagues set out to test in experiments where a-Si-based PV cells were exposed to high temperature, pulse annealing cycles at different temperatures and for different durations. They also examined how the thickness of a-Si layers would affect overall efficiency at different temperatures, including 50°C to mimic the operating temperature of PV cells, and the 90°C akin to PV/T systems.

Dr. Pearce holding a PV laminate

Their findings were reported in Solar Energy, the journal of ISES, the International Solar Energy Society (Vol 86, Issue 9, September 2012) and in Solar Energy Materials and Solar Cells (Vol 10, May 2012), a journal published by Elsevier.

Spike annealing

The experimental procedures are described in the papers, but to summarize: they spike annealed a-Si PV cells in conditions mimicking a commercial PV/T system. The preliminary findings demonstrated that annealing produced a 10.6% electrical efficiency gain.

'It's a really big deal,' says Dr. Pearce. 'We were really surprised by the gain we got with spike annealing. To put this in context, the last time I made a significant contribution to the field was a 3% increase. That was huge, and this time we're getting an increase for all the cells not in just some special situation.'

They also found that a-Si solar cells at 90°C could generate more power than commercially-produced cells if the i-layers, the part of the a-Si solar cell that absorbs light, were thicker; and this was true at both PV and PV/T operating temperatures.

This latest thin-film technology breakthrough could see more home installations

Conclusion: use thicker than normal a-Si i-layers in a PV/T system and you can get higher electrical efficiency. Bake the a-Si layers once a day and degradation under Staebler–Wronski is reversed by annealing. It would take 15 minutes to explain the temperature controls needed to do this, according to Dr. Pearce.

'We also know intuitively that the higher the temperature of the solar thermal absorber, the higher the efficiencies for the solar thermal system,' he adds. 'With the electrical and thermal efficiency gains, we're making two better mousetraps for the price of one.'

For a man who describes himself as 'an old, amorphous silicon, solar cell scientist', the recent discoveries mark a personal watershed in a three decades-long tussle with the Staebler–Wronski effect. He even did his PhD thesis on it.

'The way the industry has dealt with Staebler–Wronski has been to make a-Si PV cells abnormally thin, thinner than they should be to collect all the light. That limits the degradation so you can reach a steady state.'

The use of a-Si looks set to boost the efficiency of solar cogeneration Credit: MTU

This has led to a situation where anyone buying a-Si PV cells on the market today knows they will be around 11% efficient in converting light into energy. This will degrade to about 10% in the first month and stay around that mark for the rest of the cells' useful life.

It is a fix, but a sub-optimal one, confirms Dr. Pearce. 'We could have a 13% efficient solar cell if we could make it as thick as it wants to be to collect light and there was no degradation of its efficiency.'

As with many scientists discussing research findings, he sounds a note of caution. 'We only looked at single-junction cells, while there are double- and triple-junction cells on the market. But there's now a clear technological path for many people in the industry to move forward: 20 years from now, every roof may be made of integrated PV/T.'

Pearce's team is continuing tests and looking at more sophisticated versions of what it has been doing. 'Some of our partners are pursuing lines of research, and I would be shocked if several PV/T companies were not looking at this now. In fact, I know that one definitely is,' he adds.

Open source technologies

Round about the time that the scientific papers were published, an unnamed American company filed for a patent which, says Dr. Pearce, included data remarkably similar to his team's findings.

Asked about the intellectual property rights, he stressed that he runs the Open Solar Outdoors Test Field (OSOTF) and is a strong proponent of open source technologies of all kinds.

OSOTF is a Canada-based, fully grid-connected system that monitors the output of nearly 100 PV modules in the field.

'I don't patent stuff, I try to help everybody else and have found it very beneficial,' says Dr. Pearce. 'For example, OSOTF has more than half a million [US dollars] worth of equipment and I have not had to write any grant applications for it. Everybody donated because they wanted the data.'

The understanding was that if they gave him equipment, he would test it for free, give them their own data and also share everyone's data rolled into one set.

So now that performance analysis is available and the entire field can benefit, Dr. Pearce believes this extends not only to the solar industry but to people wishing to make informed decisions about installing systems on their own homes.

'There's now potential for every single amorphous silicon thin-film manufacturer in the world to either partner, or take over, or begin to use a-Si for solar thermal applications.'

Could people not just engineer around the techniques and file patents?

'Right now, it's published, it's out there in stone,' Pearce responds. 'So people can talk about their type of manufacturing process being a little bit better, and that's fine, but they can't take the basic idea.'

How easy would it be to transfer these technological discoveries into improved PV/T systems? 'There's no major roadblock, it's just a matter of getting it out to the public,' says Dr. Pearce.

One firm to have shown clear intent is California-based ThinSilicon, a thin-film solar cell maker that co-operated on the project and co-authored a paper.

To set the research in context, the investment climate for PV/T has thus far been lacklustre. Venture capital investment in PV/T has fallen continuously since mid-2010 though it is still above its level before the so-called 'Great Recession', according to US-based analyst Cleantech Group.

Among US companies in this field, EchoFirst of Fremont, CA, raised nearly $14 million in equity and is selling a PV/T system commercially for residential use – cogeneration for existing homes and trigeneration for new homes.

UltraSolar Technology, Santa Clara, CA has raised over $4.3 million to develop proprietary technology that converts 'wasted' heat in solar cells to electricity to increase the electrical output of PV cells or modules for residential, commercial and utility arrays.

A rooftop PV/T array Credit: Alfagy

Meanwhile, Solergy, of Oakland, CA, but with an R&D centre in Rome, Italy, is developing high concentrated PV technology for electricity alone or for cogeneration and sees its markets as being residential, commercial and industrial.

Bright spots ahead

The breakthroughs announced by Dr. Pearce and colleagues will be of extreme interest to the entire PV/T industry and to thin-film PV array makers.

'The PV industry is in the midst of wrenching change, buffeted by government incentive cuts and nose-diving prices, that has hurt solar suppliers worldwide, rocked by trade disputes among its major players, and hamstrung by a sputtering global economy,' according to Ash Sharma, director, Solar Research at IMS Research, a UK-based supplier of market research and consultancy to the global electronics industry.

'However, there are some bright spots ahead: solar installations are on the rise, technology is becoming more efficient, and a weak EU [European Union] market roiled by financial turmoil will be offset by an ascendant China and the United States.'

The average price of PV wafers in the first quarter of 2012 fell by more than 70% on an annualized basis and has been plumbing record lows, IMS said in a trends report on 17 December. Suppliers are reducing in-house manufacturing and purchasing more wafers from third-party suppliers.

Many thin-film solar cell manufacturers, particularly those who have not reached sufficient scale to be competitive, are feeling the pinch, while some are going out of business, says Dr. Pearce.

'There's two ways for these companies to compete,' he suggests. 'Get big, or partner with or buy a solar thermal firm to put their modules on solar thermal devices. It's frustrating, but I hope it's all going to work out in the end.'

Might the findings also make PV and PV/T more attractive in areas that have fewer hours of sunshine per year, and less intense light?

'A little bit,' confirms Dr. Pearce. 'Amorphous silicon has historically had higher kilowatt hours per kilowatt generation, primarily through a spectral effect. So if you're in an area that matches amorphous silicon better it can provide some benefit.'

He cited the example of snow-covered areas where the spectrum of light reflected from the snow is 'almost perfect' for the light trapping properties of a-Si.

'But I am pretty sure that this technology will produce more electricity no matter what. It will also probably produce more useful solar thermal energy, the one exception being if you are in a super-hot climate where you don't have some form of trigeneration (heat, power, cooling). If you're only producing heat and electricity and you don't need any more heat, then it won't help you.'

One obvious question about overcoming the Staebler–Wronski effect is why it took so long, when so much was apparently already understood about the techniques that Pearce and his colleagues are now perfecting.

'Everyone is in their own little ivory tower,' replies Dr. Pearce. 'Very rarely do solar electric people talk to solar thermal people, In this case, I (a solar electric guy) and Steve Harrison (a solar thermal colleague) were sharing supervision of a research student and discussing what we could do together. It turned into this fantastic partnership. Neither field would have come up with this by itself.'

The fruits of the collaboration mean solar power and thermal are also looking more like a made-on-earth marriage that could start to make inroads into markets for use in residential and commercial/municipal properties.